The hormone that keeps a family man from straying

Scientists have found that men who spend lots of time with their wives and children have lower levels of testosterone than bachelors.

The hormone controls men's aggression and sexual appetite.

The discovery by scientists in America suggests that women who want to know if their man is likely to remain faithful might one day demand to check out his testosterone levels.

Certainly the hormone has a role to play in encouraging men to devote their energies to the family, rather than looking for another partner, says the team at Harvard University.

Anthropologist Dr Peter Gray and his colleagues at Harvard measured testosterone levels in the saliva of 58 men who were either single, married, or married with children.

In all the men, hormone levels followed the normal cycle of peaking in the morning and falling over the course of the day, according to a report in the magazine New Scientist today.

But the decrease was greater in married men than bachelors, and even more pronounced in fathers.

'Fathers seem to show an even more dramatic difference from unmarried men,' Dr Gray said. He said he believed the most devoted dads, who are less likely to stray, had the lowest testosterone levels.

The scientists admit they cannot be certain whether men who start off with lower levels of the hormone are simply likely to make better fathers, or whether spending time with a family causes the drop-off.

Dr Gray believes it could work both ways. Lower levels encourage men to spend time with their family and being in a family may lower the hormone levels, he suggested.

Experts claim that ageing in men naturally leads to a dramatic drop in levels of testosterone, similar to the loss of oestrogen experienced by women when they go through the menopause.

Scientists say it is clear that testosterone has a major impact on human behaviour, though more research is needed to work out all of its effects.

They believe that stressful events such as bereavement, divorce or lifethreatening illnesses could also send testosterone levels temporarily plummeting.

Research suggests that men could experience some benefits by taking testosterone once they reach middle-age.